1. Field of the Invention
This invention relates to an improved process for bonding polymeric sheet materials together, and particularly to a process for bonding a sheet of plexifilamentary polymeric material to a second sheet of polymeric material. The process is useful in the fabrication of porous plexifilamentary tubing which can be used for agricultural purposes, such as irrigation.
2. Description of the Prior Art
Processes for forming tubing from heat-sealable materials are known in the art. U.S. Pat. No. 2,522,346 discloses the manufacture of tubing from rolls of material in which two plies of heat-sealable film are separated by a liner of non-heat sealable cellulosic material. Wide rolls of the heat-sealable film are first sliced and then brought into contact with a heated plate or open flame. The heating of the ends of the rolls causes contacting edges of the heat-sealable film to become united, and the linear material prevents the union of those portions of the film separated by the liner. In this manner, tubing of any desired width may be prepared.
U.S. Pat. No. 2,916,053 discloses a process for forming tubing from superposed layers of thermoplastic sheet material using a sealing heat internally equipped with a conventional resistance type electric heating element.
U.S. Pat. No. 3,538,912 discloses a method of bonding layers of porous and liquid absorbent non-woven cellulosic material using an intermediate layer of thermoplastic material. This process requires interleaving the thermoplastic material between the superposed non-woven sheets. The bonds formed, however, are of marginal strength.
British Pat. No. 993,092 discloses a process for bonding two thermoplastic films together by applying a strip of hot thermoplastic material to one film and allowing the heat of the applied material to fuse the two sheets together. This patent, however, does not deal with a process for making tubing nor does it deal with the problem of bonding two layers of material together when one of those layers is a plexifilamentary sheet.
The problems encountered in bonding a sheet of fibrous, non-woven plexifilamentary polymeric materials either to another sheet of the same material or to a sheet of a second polymeric material are entirely different from those encountered in bonding two solid thermoplastic films together. As with all fibrous materials, the plexifilaments in such a sheet structure react to the heat used in the sealing process and "neck down" to such an extent that the mere act of applying heat to the sheets, for the purpose of effectuating a bond between the two layers of material, actually creates a rupture line at the edge of the heat seal.
The situation is even more complicated when the process is used to produce a tube which can be used in trickle irrigation. While a small hole, which tends to plug quickley with water sediments, can be tolerated in such a tube, a hole with a rupture line around it, which will increase in size due to the pressure exerted on the weakened fibers in the region of the hole, cannot be tolerated. Such a hole, even a microscopic hole which allows an almost invisible squirt of water to escape, can leak an order of magnitude more water than intended for each plant and thereby reduce proportionally the amount of water provided to plants downstream from the hole. Furthermore, for outdoor use, the bond formed should, preferably, be resistant to ultraviolet degradation and the tubing formed should be resistant to the formation of kinks. Furthermore, because of its inert nature and relative cheapness, polyethylene is a preferred material for irrigation tubing. Plexifilamentary polyethylene sheets are particularly difficult to bond together or to bond to solid polyethylene sheets.